Polyolefins are a class of polymers prepared from simple olefins. Methods of preparing polyolefins Ziegler-Natta polymerization catalysts, which use a transition metal halide to polymerize vinyl monomers and provide a polymer having an isotactic stereochemical configuration.
Typically, two types of Ziegler-Natta catalyst systems are used in the polymerization or copolymerization of olefins. The first, in its broadest definition, comprises TiCl3-based catalysts components obtained by reduction of TiCl4, in combination with an alkyl aluminum compound, such as diethylaluminum chloride (DEAC). These catalysts provide polymers of high isotacticity, but they have a low activity resulting in the presence of large amounts of catalyst residue in the polymers.
The second type of Ziegler-Natta catalyst system comprises a solid catalyst component, wherein a magnesium dihalide supports a titanium compound and an internal electron donor compound. A variety of internal electron donor compounds are added during catalyst synthesis in order to maintain a high selectivity for an isotactic polymer product. Conventionally, when higher polymer crystallinity is desired, an external donor compound also is added during the polymerization reaction.
The internal electron donor is an important component in a propylene polymerization catalyst. It controls catalyst activity and hydrogen response, as well as the composition of the resulting polymer. Different internal donors result in catalysts with vastly different performance characteristics. The discovery of high performance internal donor is one of the most active research activities among the major propylene catalyst producers.
Currently, three internal donor families are being used to produce polypropylene (PP) catalysts:
(a) Dialkyl phthalates, wherein the most widely used compounds are di-t-butyl phthalate and di-n-butyl phthalate. The phthalate-containing catalysts have high activities and produce PP having a high isotacticity index and a medium molecular weight distribution;
(b) Dialkyl succinates. The succinate-containing catalysts have high activities and produce PP having high isotacticity index and broad molecular weight distribution; and (c) 1,3-Diethers. The diether-containing catalysts have super high activities and produce PP with high isotacticity.
To enhance catalyst performance, substantial research efforts have been undertaken by the polyolefin industry to discover new families of high performance internal donor compounds, but with limited success. The discovery of high performance internal donors capable of producing polyolefins with the desired properties is still one of the most important research goals of the polypropylene industry.